1. Field of the Invention
The present invention generally relates to a gas sensor using an oxygen-ion conductor, and particularly to a gas sensor having at least one cavity formed in the oxygen ion conductor and an internal electrode formed inside the cavity wall made of the oxygen-ion conductor. More Specifically, the present invention relates to an e.m.f. cell (or rather electromotive force cell) formed in such a sensor that is temperature-controlled by detecting an internal resistance of the e.m.f. cell that is used as an accurate reference oxygen source for detecting a low concentration of a specific gas such as O2, NOx, HC, H2, CO, CO2 and H2 O.
2. Description of the Related Art
A gas sensor using an oxygen-ion conductor, having at least one cavity or chamber formed inside the oxygen-ion conductor and a diffusion resistance hole capable of communicating a gas mixture from outside into the cavity is known for measuring an amount or concentration of a specific gas component contained in the gas. For instance, U.S. Pat. No. 5,700,367 to Yamada et al relates to such a sensor having one cavity. Published European Patent Applications (EP 0810430 A2 and EP 0859232 A2) relate to such a gas sensor having two cavities.
When a gas mixture containing a specific gas component such as NOx and oxygen is introduced into a first cavity of the sensor having two cavities, it is understood that oxygen contained in the mixture gas that enters into the first cavity is pumped out through an oxygen-ion pumping cell so that the specific gas that flows from the first cavity to the second cavity can be measured accurately based on a small constant oxygen concentration (offset level) and a variation in the oxygen concentration. This variation is caused by the specific gas component decomposing or burning inside the second cavity. The gas component amount (or concentration) is determined based on the amount of the oxygen produced or decreased by such decomposition or burning and the offset level of the amount of oxygen which enters into the second cavity.
However, in order to precisely or rather accurately determine the specific gas component amount on such a small order, for instance, of less than 500 PPM or less than 100 PPM by the sensor, it is understood that the total amount of oxygen entering into the second cavity other than the oxygen produced or decreased as a result of decomposition or burn of the gas component in the second cavity should be as small as possible. This is because a small amount of the specific gas component can be notably compared with the total amount of the oxygen in the second cavity. In other words, oxygen not entering through the diffusion resistance hole formed between the cavities but rather entering through other portions such as a third cavity or an internal oxygen-reference electrode formed in the third cavity should be minimized.
In addition, the constant oxygen partial pressure detected at the internal oxygen reference electrode or in the internal oxygen reference cavity becomes critical when accurate measurement of the small amount of the specific gas component is required. This is because the oxygen partial pressures in the first cavity and/or the second cavity must be referred or compared with the constant oxygen partial pressure at the internal reference electrode or in the internal-reference cavity. In other words, the e.m.f. voltage that determines the partial oxygen pressure in the first or second cavity is importantly determined based on the Nernst equation by reference to the oxygen partial pressure appearing at the oxygen-reference electrode or in the internal oxygen-reference cavity.
Therefore, the present inventors consider that the measurement accuracy of the specific gas component depends on how good or how precisely such an e.m.f. voltage cell or the cell that determines the oxygen variation in the first and/or second cavity is designed or structured.
It is therefore an object of the present invention to provide a gas sensor which is capable of accurately measuring a concentration of a specific gas component such as NOx, O2, CO2, SOx, H2O, CO, HC and H2 contained in a gas mixture of interest such as an exhaust gas emitted from an automobile internal combustion engine.
It is another object of the present invention to provide a gas sensor structure which is capable of accurately detecting a temperature of the gas sensor and of effectively controlling the sensor temperature.
There is provided in accordance with a first aspect of the invention., as may be referred in FIGS. 1 and 2, a gas sensor (1) using two oxygen ion conductor substrates (21),(23): comprising,
a cavity (5) formed between the oxygen-ion conductors (21),(23);
a diffusion hole (3) formed as an entrance to the cavity (5);
an internal electrode (23a) formed on an internal wall of one of the oxygen-ion conductor (23) and another electrode (23b) formed on the oxygen-ion conductor (23) spaced from the internal electrode (23a), forming an electromotive force cell that detects an oxygen partial pressure in the cavity (5);
a metallic wire (23c) formed along the oxygen ion conductor (22) so as to connect one end of the metallic wire (23c) to the internal electrode (23a);
another metallic wire (23d) formed along the oxygen ion conductor (22) so as to connect one end of the another metallic wire (23d) to the another electrode (23b); and
a heater (30) for heating and activating the oxygen-ion conductor (22);
wherein a combined resistance (Rpvs) determined by applying a stepped current between the other end of the metallic wire (23c) and the other end of the another metallic wire (23d) at a high temperature of 500-900xc2x0 C. that activates the oxygen-ion conductor (22) so as to transfer oxygen ions has a value not less than 2.6 times greater than that of a lead-resistance (Rlead) in total of the two metallic wires (23c),(23d),
the lead-resistance (Rlead) being the value measured along the two metallic wires (23c),(23d) excluding a cell internal resistance of the electromotive force cell appearing across the electrodes (23a),(23b), and
the combined resistance (Rpvs) including the lead-resistance (Rlead) and the cell internal resistance.
In the sensor according to the invention, an accuracy range of temperature control in the temperature range of 500-900xc2x0 C. becomes less than 5xc2x0 C. If the combined resistance is not less than 4 times greater than the lead-resistance, the accuracy range advantageously becomes less than 2.5xc2x0 C.
A temperature variation lowers to less than 5xc2x0 C. if the combined resistance is higher than 50 ohms and the lead-resistance is designed to less than ⅓ of the combined resistance. The very low temperature variation is attained when the combined resistance is designed to higher than 80 ohms at 700xc2x0 C. and the lead-wire resistance is less than ⅕ of the combined resistance. It is important to reduce the total lead wire resistance to be less than ⅓ of the combined resistance (Rpvs) of an e. m. f. cell (or oxygen detection cell) incorporated in the NOx sensor which is further described in the detailed description.
The above sensor (1) may further include a third oxygen-ion conductor (25) spaced from one of the oxygen-ion conductor (23); a second cavity (9) formed between the third oxygen-ion conductor (15) and the oxygen ion conductor (23); a second internal electrode (25a) formed on the third oxygen-ion conductor and inside the second cavity (9); another second electrode (25b) formed on the third oxygen-ion conductor and outside the second cavity (9); and a second diffusion hole (7) formed between the first cavity (1) and the second cavity (9).
In a second aspect of the invention, there is provided, as may be referred in FIG. 9, an improved gas sensor (51) that differs from the sensor (1) previously described, in that the electrodes (23b),(25b) for oxygen-reference in FIG. 9 are placed in a common cavity (53) providing the same internal oxygen-partial pressure to the electrodes (23b),(25b).
This second aspect of the invention is also important in providing a high accuracy measurement sensor, because the first cavity (5) and the second cavity (9) are interrelated through the second diffusion hole (7) and need the same oxygen reference atmosphere that is formed inside the common cavity (53).
The third aspect of the invention relates to a distance of the electrode (23b) for oxygen reference and the metallic wire (23c) away from the second cavity (9). If the distance of the electrode (23b) or the metallic wire (23c) are too close to or less than 0.5 mm away from the cavity (9), leakage of oxygen occurs and causes a offset second current (Ip2 offset) to flow unnecessarily high rendering inaccurate measurement of a specific gas concentration contained in the oxygen-containing gas.